Collection Tubes Apparatus, Systems, and Methods

ABSTRACT

Methods of producing collection tubes are presented. The methods include providing a separator substance that can rapidly polymerize in a short time to a desired hardness and disposing the separator substance within the lumen of the tube. The separator substance is formulated to have a density between an average density of a serum fraction of whole blood and a cell-containing fraction of whole blood, and to be flowable with whole blood. Upon centrifugation of a tube having blood, the separator substance forms a barrier between the whole blood fractions. The tube and barrier maintain stability of one or more analyte levels, including potassium and glucose, within 10% of their initial values before centrifugation for at least four days.

This application is a divisional of U.S. patent application having Ser.No. 13/174957 filed Jul. 1, 2011 which is a divisional of U.S. patentapplication having Ser. No. 12/271610 filed Nov. 14. 2008 now U.S. Pat.No. 7,971,730 issued Jun. 15, 2011, which is a continuation-in-part ofU.S. patent application having Ser. No. 11/933839 filed on Nov. 1, 2007now U.S. Pat. No. 7,673,758 issued Mar, 9, 2010; which is acontinuation-in-part of U.S. patent application having Ser. No.11/499,436 filed on Aug. 4, 2006 now U.S. Pat. No. 7,674,388 issued Mar.9, 2010; which claims priority to U.S. provisional patent applicationSer. No. 60/707,299 filed on Aug. 10, 2005; and this application claimsthe benefit of priority to U.S. provisional application having Ser. No.61/028,426 filed on Feb. 13, 2008. These and all other extrinsicreferences are incorporated herein by reference in their entirety. Wherea definition or use of a term in an incorporated reference isinconsistent or contrary to the definition of that term provided herein,the definition of that term provided herein applies and the definitionof that term in the reference does not apply.

FIELD OF THE INVENTION

The field of the invention is separation technologies.

BACKGROUND

Analysis of blood samples often requires separation of whole blood intoa serum fraction and a cell-containing fraction. It is well known in theart that whole blood separation can be carried out throughcentrifugation by disposing whole blood into a blood collection tube,placing the tube into a centrifuge, and spinning down the blood.

Unfortunately, once the blood separates, the fractions of the wholeblood can remix causing contamination of the fractions throughdiffusion, agitation, sample extraction, or other undesirableinteraction. Ideally, the two fractions should remain isolated to ensureno contamination occurs when accessing the desired fraction.Furthermore, the analytes of the blood should maintain stability afterseparation over extended periods of time to provide for storage,shipping, or late term analysis.

Any system that isolates the fractions of whole blood must include aseparator substance having a suitable density within the tube. Suitabledensities are about 1.04 g/cm³ and are between the density of theheavier cell-containing phase and the density of the lighterserum-containing phase. When whole blood is added to the tube and thetube is centrifuged, the separator substance migrates to between thefractions isolating the two fractions from each other. An examplecollection tube using a gel as a separator substance and that isflowable with whole blood can be found in U.S. Pat. No. 4,946,601 toFiehler. An example separator substance that is also flowable with wholeblood can be found in U.S. Pat. No. 6,248,844 and U.S. Pat. No.6,361,700 to Gates et. al. In those patents the substance is a polyestercurable to a desired viscosity.

Although providing a flowable substance allows for separating thefractions of whole blood, flowable substances have severaldisadvantages. A flowable substance remains flowable even aftercentrifugation which results in a risk of contamination of the sample ifproper care is not taken to keep the sample suitably still and protectedfrom agitation. For example, it is known to use a thixotropic gel in ablood collection tube where the gel can still flow after centrifugation.Additionally, known substances lack the ability to maintain analytes(e.g., potassium and glucose) at acceptable levels over extended periodsof time (e.g., for at least three days).

U.S. Pat. No. 4,818,418 to Saunders discusses the use of a thixotropicgel in blood collection tubes. The problem with thixotropic gels,however, is they do not form a sufficiently permanent separation barrierbetween the fractions of whole blood. When a sample is extracted fromthe tube with a pipette, the substance can contaminate or plug thepipette if it touches the substance due to the flowable nature of thesubstance. If the substance is formulated or configured with a highviscosity to provide a sufficiently solid or permanent barrier toovercome the previous disadvantages, then the substance is no longersuitably flowable with whole blood resulting in prohibitive centrifugetimes. Short centrifuge times are critical in life or death situationswhere a blood analysis result is required quickly.

An alternative approach taken by collection tube manufactures is toprovide moveable solid barriers. Examples of suitable solid substancesinclude the intermediate density polymers found in U.S. Pat. Nos.3,647,070 where polymer spheres form the barrier layer. U.S. Pat. No.5,266,199 describes a tube-and-ball valve that controls separation ofthe serum from the cell-containing phase. However, such physicalbarriers do not provide a sufficient seal between the fractions and areoften either incomplete and tend to leak, or impracticable for othervarious reasons.

These and other solutions for whole blood separation lack the necessaryfeatures to ensure the separated factions of whole blood are effectivelyprotected against contamination due to undesirable sample interactionswhile supporting short centrifugation times. Furthermore, knownseparation technologies fail to maintain stability of analytes,especially potassium and glucose, over extended periods of time. Thus,there is still a need for liquid separation technologies in which theseparation layer can be hardened and preserve stability of analytes.

SUMMARY OF THE INVENTION

The present invention provides apparatus, systems and methods in which acollection tube includes a separator substance that maintains potassiumlevels and glucose levels within acceptable thresholds for extendedperiods of time. In one aspect of the inventive subject matter,potassium levels are stable within 10% of an initial level beforecentrifugation and glucose levels are stable within 5%. Furthermore,preferred collection tubes are able to keep analytes stable for at leastfour days, or even up to five days.

Another aspect of the inventive subject matter includes methods ofproducing collection tubes. A separator substance is disposed within thetube where the substance is formulated aid in keeping analytes stableover extended periods of time. The tube can also be sterilized usinggamma radiation or heating the tube to at least 250 degrees Celsius.

Various objects, features, aspects and advantages of the inventivesubject matter will become more apparent from the following detaileddescription of preferred embodiments, along with the accompanyingdrawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1A is a side perspective view of a blood collection tube having apolymerizable separator substance that can harden.

FIG. 1B is a side perspective view of the blood collection of tube ofFIG. 1A after adding whole blood.

FIG. 1C is a side perspective view of the blood collection tube of FIG.1B after centrifugation.

DETAILED DESCRIPTION Collection Tube

In FIG. 1A blood collection tube 100 generally comprises tube 110, plug120, and separator substance 150, where tube 110 has lumen 115. Tube 110is preferably manufactured out of a suitably rigid material to support avacuum within lumen 115. Example materials include hard plastics, glass,or other similar materials. Lumen 115 is of sufficient volume to hold adesirable sample of whole blood or liquid. Typical volumes range from afew ml to 10 ml or greater. Plug 120 fits sufficiently snug into tube110 to maintain the vacuum within lumen 115. It is contemplated thatplug 120 is manufactured to provide a color code or other indicationthat separator substance 150 is disposed within lumen 115. An example ofan acceptable tube that can be used to produce collection tube 100includes the Vacutainer® specimen collection products developed byBecton, Dickenson and Company (Franklin Lakes, N.J. USA 07417).

The term “tube” is used euphemistically to refer to vessels having acavity. Although a preferred embodiment includes tube 110, one shouldappreciate that other vessels having a cavity can also be used whilestill falling within the scope of the inventive subject matter. Forexample, it is contemplated that collection tube 100 could be replacedwith other vessels that can contain a liquid or optionally support avacuum. Alternative examples of vessels include flasks, jars, beakers,bottles, blood collection bags, or phials. Vessels beyond mere tubesalso have utility when the inventive subject matter is applied toalternative markets beyond blood collection.

In a preferred embodiment, collection tube 100 is produced by disposingseparator substance 150 within lumen 115, and introducing a vacuumwithin lumen 115 in preparation for sale. It is also preferred that nomore than about 1 ml, or about 2 grams, of separator substance 150 isdisposed into lumen 115 for a typical 10 ml collection tube. It iscontemplated that other amounts, more or no more than 1 ml, could alsobe used to fit a specific use case. For example a smaller version oftube 110 would require less of separator substance 150, while a largerversion might require more to make an adequate sealed barrier.

In some embodiments, tube 100 is sterilized before tube 100 is sold. Forexample, tube 100 can be sterilized using gamma radiation before addinga preferred photopolymer. Another example of sterilization includesheating tube 100 to at least 250 degrees Celsius after adding thepreferred photopolymer. It is also contemplated that other methods ofsterilization could also be used without departing from the inventivesubject matter. Other forms of sterilizations beyond thermal orradiation based sterilization could include chemical sterilization.

An optional vacuum can be introduced by simply decompressing the volumeof lumen 115 by using a suitable pump. The term “vacuum” within thecontext of this document means a partial vacuum having a pressure lowerthan the pressure external to tube 110.

It is also contemplated that a user could add one or more separatorsubstances to a collection tube after purchase, as opposed to having aseparator substance pre-disposed within the tube.

FIGS. 1B represents an exemplary embodiment of a blood collection tubeafter introduction of blood 140, and before centrifugation. Althoughblood 140 is shown on top of separator substance 150, the two might havecharacteristics in which they are free to flow or mix.

FIGS. 1C represents an exemplary embodiment of a blood collection tubeafter centrifugation. During centrifugation blood 140 separates intoserum fraction 160 and cell-containing fraction 170. When separatorsubstance 150 has a density that is intermediate to that of serumfaction 160 and cell-containing fraction 170, it migrates between thetwo fractions during centrifugation, thereby isolating fractions 160 and170 from each other. Separator substance 150 can then be rapidlyhardened through polymerization when triggered by a suitable energysource.

Separator Substance

Preferably separator substance 150 rapidly hardens during finalpolymerization to a hardness that is resistant to penetration by apipette, to decanting, or even to freezing. Preferred substances aresolid with respect to a probe, possibly a pipette.

Hardness can be measured using any suitable hardness scale including oneof the Shore hardness scales. The Shore 00 hardness scale is used tomeasure the hardness of soft substances including gels or foams. TheShore A hardness scale is used to measure the hardness of substanceshaving an intermediate hardness including rubbers. The Shore D hardnessscale is used to measure the hardness of harder substances includingplastics. Although the preceding Shore hardness scales are used fordifferent various substances, the scales all overlap at the low end oftheir spectrums. Therefore, a value of 10 on the Shore D scale is harderthan a value of 10 on the Shore A scale which in turn is harder than avalue of 10 on the Shore 00 scale. Separator substance 150 is preferablyformulated to harden to at least 1 on the Shore 00 hardness scale. Morepreferred embodiments of separator substance 150 harden further to atleast 10 on the Shore A hardness scale. In yet other embodimentsseparator substance 150 harden even further to at least 10 on the ShoreD hardness scale.

Within the context of this document, the term “rapidly hardens” means toharden to at least 1 on the Shore 00 hardness scale within at least 10minutes. One of the aspects of the inventive subject matter isappreciating that a shorter time to harden can be advantageous over alonger timer. Having separator substance that hardens within a fewminutes, for example, could be important for a hospital to analyze asample in a critical life or death situation. In preferred embodiments,the time to harden is no more than 5 minutes, more preferably no morethan 1 minute, and most preferably no more than 10 seconds.

The hardened barrier of preferred separator substances adheres to thewalls of lumen 115 substantially sealing the cell-containing fractionand protecting the fractions from contamination due to diffusion,agitation, sample extraction, or other undesirable interaction. Inpreferred embodiments the final thickness of the barrier is no more than5 mm.

Separator substance 150 is preferably a biocompatible organic polymer.Among other things, biocompatibility means that the separator substance150 does not interfere with or alter characteristics of the substancesbeing tested. In the case of blood, for example, the separator substance150 should not interfere with pH, enzyme activities, or withconcentrations of pigments, proteins, gases, or any other analytes.

In yet other embodiments, it is contemplated that the substance couldinclude a component that intentionally reacts with the sample beingseparated. For example, the separator substance could include acoagulant, blood thinner, or other substance that interacts with wholeblood.

In blood separation tubes, the separator substance 150 should have adensity of between about 1.01-1.09 g/cm³, and most preferably about 1.04g/cm³. Unless the context dictates otherwise, all ranges herein are tobe interpreted as being inclusive of their endpoints.

An acceptable separator substance can include a polyester backbonesimilar to those described in U.S. Pat. Nos. 6,361,700 and 6,248,844,both of which are incorporated by reference herein. Polymerization ispreferably carried out to achieve the desired density of between about1.04-1.06 g/cm³. However, and in contrast to the methods andcompositions provided in the '700 and '844 patent, polymerization is notrun to completion but stopped using a polymerization terminator (e.g.,using radical quenchers, catalyst complexing agent, etc.) in a minimumamount effective to stop further polymerization.

As the sample contacts the incompletely cured polymer (separatorsubstance 150), it is contemplated that the polymerization terminator isdiluted to a concentration that allows the polymerization to bere-initiated. Prior to re-initiation, blood 140 is separated in thecontainer by centrifugation, which will leave cell-containing fraction170 in the bottom portion of tube 110 and serum fraction 160 in theupper portion of tube 110, wherein both fractions are separated by theincompletely cured polymer (separator substance 150). Re-initiation ofpolymerization may be assisted by irradiating the polymer with UV lightor other suitable energy source. Thus, it should be appreciated that thepolymeric is additionally cured after the separation is completed andthe so separated serum can then be accessed without contamination of apipette, decanted, or even frozen. Moreover, it should be recognizedthat the final cured barrier layer is substantially permanent (i.e.,stable over several days, or even weeks).

While it is generally acceptable that collection tube 100 include apolyester polymer as separator substance 150, it should be noted thatthe exact nature of the polymeric material is not limiting to theinventive subject matter, and that numerous alternative polymers arealso suitable. Indeed all known polymers suitable for whole bloodseparation are deemed appropriate for use herein, including silicon oil,polyamides, olefinic polymers, polyacrylates polyesters and copolymersthereof, polysilanes, and polyisoprenes. To achieve a desired initialdensity (typically between about 1.03 and 1.05), it is contemplated thatthe density may be adjusted by virtue of molecular composition, as wellas by inclusion of appropriate filler material (e.g., silica, latex, orother inert material). For example, suitable polymeric materials aredescribed in U.S. Pat. Nos. 3,647,070, 3,920,557, or 3,780, 935, or inEP 0 928 301 or 0 705 882, which are incorporated by reference herein.Furthermore, it is contemplated that the serum separators may includeadditional materials and/or reagents to achieve a desired bio-reactivepurpose. For example, the separators presented herein may include EDTA,heparin, citrate, dextrose, etc. It should be noted that the term“serum” is used herein to also include plasma, and other substantiallycell free fluids derived from whole blood.

Depending on the particular material, it is contemplated that the modeand/or mechanism of polymerization to the separator polymer may varyconsiderably, and all know manners of polymerization are deemed suitablefor use herein. For example, contemplated polymerizations includevarious radical or cationic polymerizations (e.g., using photolabilecompounds, radical starters, etc.), condensation polymerizations,esterifications, amide formation, etc. Thus, reactive groups willespecially include acid groups (and most preferably mono- anddicarboxylic groups), conjugated diene groups, aromatic vinyl groups,and alkyl(meth)acrylate. Such exemplary reactive groups and reactionconditions are described, for example, in U.S. Pat. No. 6,989,226, whichis incorporated by reference herein. It should furthermore beappreciated that the reactive groups can be coupled to the terminus of apolymer as end groups as described in WO 99/64931, which is incorporatedby reference herein, or that the reactive groups may be provided aspendant groups (e.g., as described in U.S. Pat. No. 5,336,736,incorporated by reference herein).

It is generally preferred that polymerization is fully supported byreactive groups on pre-polymer, but additional reagents may also besuitable, including radical starters, including those described in U.S.Pat. No. 5,582,954, 4,894,315, and 4,460,675, which are incorporated byreference herein. Additionally contemplated separator substances alsoinclude those that provide a crosslinking group to the polymer such thatthe polymer has reactive groups that react with a bifunctionalcrosslinker (e.g., ethylenically unsaturated compounds) to thereby formcrosslinked polymers. Yet additional contemplated separator substancesalso include those having promoters that accelerate polymerization.

An acceptable example of separator substance 150 includes a substanceknown as “M1L1A1” co-developed by the University of Maryland andUniversity of California Irvine. M1L1A1 is a polymeric separatorsubstance comprising the following: (M1) a monomer Trimethylolpropanepropoxylate triacrylate from Sigma-Aldrich Cat. No. 407577, (L1) CYTECAliphatic Urethane Acrylate EBECRYL 230 from Cytec Industries, Inc., and(A1) Additol BDK, 2,2-Dimethoxy-1,2-diphenyl-ethan-1-one also from CytecIndustries, Inc. Additionally, M1L1A1 has desirable properties for usewith whole blood including an adjustable density by adding fumed silica,it is flowable in whole blood when centrifuged, thixotropic, a hardnessgreater than 10 on the Shore A hardness scale after polymerization,hardens in no more than 10 seconds under exposure to UV light,biocompatible with whole blood, and forms a hardened seal impermeable tothe cell-containing fraction of whole blood and that is resistant topenetration of a pipette. M1L1A1 hardens under a UV light source thatradiates light in the wavelengths from 10 nm to 450 nm. A preferred UVlight source radiates in the range 250 nm to 400 nm. All suitable energysources are contemplated for triggering polymerization. It iscontemplated that an existing centrifuge having a UV source can be usedto polymerize the separator substance, or a centrifuge can incorporate asuitable energy source capable of triggering polymerization.

Preferably, the temperature of the collection tube contents changes byno more than 10 degrees Celsius during polymerization; more preferablyby no more than 5 degrees Celsius. Short exposure times ensures thesample will maintain appropriate pigment levels, gas levels,temperatures, protein levels, or other characteristics associated withwhole blood.

Preferred separator substances, including photopolymers such as M1L1A1,have additional desirable properties beyond those discussed above. Forexample, desirable substances are substantially transparent. Aftercentrifugation, the transparency of separation barrier allows an analystor technician to visually determine the completeness of separation. Thetechnician can easily observe if red blood cells, or other matter, aretrapped within the substance. Additionally, it is contemplated that thesubstances can be formulated to have a desirable color (i.e., caninclude dyes) to aid in identifying tubes or to clearly indicate thelocation of the separation barriers between two or more factions ofblood. It is specifically contemplated that separator substances canhave a color that corresponds to a coded cap of a collection tube (e.g.,green, gold, yellow, etc . . . ).

Analyte Stability

Preferred separator substances preserve the stability of one or moreanalytes of the serum or cell-containing fraction of a blood sample forextended periods of time. Maintaining stability of analytes allows longterm storage, shipping of samples, or delayed analysis. For example,blood samples can be collected in remote locations and then sent to alab located days, possibly weeks away.

In a preferred embodiment, a value of an analyte changes by less than10% over an extended period of time after centrifugation relative to avalue before centrifugation. In more preferred embodiments, analyteschange by less than 5%, and yet more preferably by less than 3%, andeven yet more preferably by less than 1%.

As used herein “extended period of time” is considered to be at leastthree days and more preferably at least four days. In more preferredembodiments a separator substance maintains stability of an analyte forfive or more days. Unless a contrary intent is apparent from thecontext, all ranges recited herein are inclusive of their endpoints, andopen-ended ranges should be interpreted to include only commerciallypractical values.

Preferred embodiments utilizing the contemplated separator substancealso maintain analyte stability across extreme environmental conditions.For example, in a preferred embodiment, analytes stability is maintainedacross a freezing-thawing event.

Analytes of particular interest include potassium or glucose. Preferredtubes maintain potassium levels within 10% for at least four days aftercentrifugation. Additionally, the substances preferably maintain glucoselevels within 5% for at least four days after centrifugation.

Several studies were conducted by the Applicant to compare and contrasta commercially available collection tube (i.e., BD Vacutainer with PST™gel and Lithium Heparin) with a similar collection tube having M1L1A1added as a separator substance. The results of one the studies conductedby the Applicant are presented in Table 1 below. The study includedtaking measurements of initial levels of analytes immediately aftercentrifugation (see columns “Init.”) and comparing the measurementstaken after storage, (see columns “1 Day” and “5 Day”). Additionally,measurements were obtained after a freezing-thawing event (see columns“Freeze-Thaw”). One should note that as used herein an “initial level”of an analyte should be considered to be the level of an analyte justafter centrifugation as measured within a reasonable time frame toconduct an analysis.

Preferred studies comparing and contrasting collection tubes includecollecting statistics from at least ten collection tubes, measuringanalyte levels periodically, and averaging the results for each analyteperiod. Such studies utilize nominal conditions including roomtemperature (e.g., about 20 degrees Celsius), lack of externalagitation, or other external influences.

TABLE 1 BD Vacutainer with BD Vacutainer with PST ™ gel and PST gel,Lithium Lithium Heparin Heparin, and M1L1A1 1 5 Freeze - 1 5 Freeze -Analytes Init. Day Day Thaw Init. Day Day Thaw Sodium 142 139 140 143140 140 142 143 Potassium 3.7 3.7 4.1 4.5 3.6 3.6 3.7 3.8 Chlorides 106104 105 108 106 105 107 108 CO₂ 29 27 26 22 27 25 25 22 Glucose 102 10093 91 104 105 102 106 Urea 16 15 16 16 17 16 16 16 Nitrogen Creatinine0.6 0.8 0.8 0.6 0.7 0.8 0.8 0.5 Calcium 9.4 9.3 9.5 9.7 9.2 9.4 9.5 9.6Total 6.8 6.8 6.4 7.0 6.8 7.0 6.7 7.0 Protein Albumin 4 4 4 4 4 4.1 4.14 Alk. Phos. 37 39 36 38 36 38 39 37 AST 17 15 19 26 17 17 18 16 ALT 1516 19 16 16 17 17 13 Total 0.6 0.7 0.6 0.5 0.5 0.6 0.6 0.4 Bilirubin

As can be seen in Table 1, a collection tube utilizing a preferredseparate substance (e.g., M1L1A1) maintains analytes at stable levelsover extended periods of time and across extreme conditions. Forexample, potassium levels are maintained within 3% of an initial levelafter centrifugation for at least five days. Additionally, glucoselevels are maintained within 2% of an initial level after centrifugationfor at least five days. It should also be noted that the levels ofanalytes are also maintained across a freeze-thaw event. Thecommercially available tube lacks such features with respect topotassium and glucose. One should also note that the commerciallyavailable tube was unable to maintain levels of AST across thefreeze-thaw event while the tube having a preferred substance was ableto maintain AST levels.

Alternative Embodiments

Although the preferred embodiment of the inventive subject matterprimarily focuses on blood collection tubes, one should recognize thatthe systems, apparatus, and methods presented herein can be applied toalternative markets beyond blood collection tubes. Similar techniques tothose disclosed herein can also be employed to separate nearly any fluidhaving more than one constituent phase. For example, a separatorsubstance can be provided to separate fluids including urine, watersamples, oil, wine, or other multi-phase fluids. For fluids having morethan two phases, it is contemplated that a collection tube can containmore than one separator substances which are used to separate at leastthree phases of the fluid.

It should be apparent to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. The inventive subjectmatter, therefore, is not to be restricted except in the spirit of theappended claims. Moreover, in interpreting both the specification andthe claims, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps may be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification claims refers to at leastone of something selected from the group consisting of A, B, C . . . andN, the text should be interpreted as requiring only one element from thegroup, not A plus N, or B plus N, etc.

What is claimed is:
 1. A sample collection tube, comprising: a tube having a lumen; a first separator substance disposed within the lumen, wherein the first separator substance is formulated to isolate a first phase of a sample of whole blood from a second phase of the sample of whole blood upon centrifugation, the first separator substance comprising a curable compound with a density between an average density of the first phase and the second phase and having sufficient reactive groups to form a first solid crosslinked composition within ten minutes of exposure to a suitable energy source; and wherein the first solid crosslinked composition, upon formation, provides a solid barrier seal between the first phase and the second phase.
 2. The tube of claim 1, further comprising a second separator substance formulated to migrate between a third phase of the sample of whole blood and at least one of the first phase and the second phase, wherein the second separator substance comprises a curable compound having sufficient reactive groups to form a second solid crosslinked composition between the third phase and the at least one of the first phase and the second phase within ten minutes of exposure to a suitable energy source.
 3. The tube of claim 1, wherein after centrifugation and after formation of the solid crosslinked composition, the tube maintains a measured level of at least one analyte within 10% across a freeze-thaw event, the analyte comprising at least one of potassium, glucose and AST.
 4. The tube of claim 3, wherein the tube maintains stability of the at least one analyte for at least four days.
 5. The tube of claim 3, wherein the tube maintains the measured level within 5% across the freeze-thaw event.
 6. The tube of claim 1, wherein the solid crosslinked composition is solid with respect to a probe.
 7. The tube of claim 1, wherein the solid crosslinked composition comprises a hardness of at least 1 the Shore 00 hardness scale after formation.
 8. The tube of claim 1, wherein after centrifugation the separator substance lacks visibly trapped cells.
 9. The tube of claim 1, wherein the solid crosslinked composition is resistant to freezing.
 10. The tube of claim 1, wherein the solid crosslinked composition is resistant to decanting.
 11. The tube of claim 1, wherein the solid crosslinked composition comprises a substantially permanent barrier.
 12. The tube of claim 1, wherein the temperature of the collection tube changes by no more than 10 degrees Celsius during formation of the solid crosslinked composition.
 13. The tube of claim 1, wherein a temperature of the collection tube changes by no more than 5 degrees Celsius during formation of the solid crosslinked composition.
 14. The tube of claim 1, wherein the first phase is a serum phase comprising plasma.
 15. The tube of claim 1, wherein the first phase is a substantially cell free fluid phase.
 16. The tube of claim 1, wherein the first solid crosslinked composition has a thickness of no more than 5 mm.
 17. A method of separating a liquid sample into a first fraction and a second fraction using a collection tube and a separator substance, comprising: obtaining a collection tube that contains a curable composition and adding the liquid sample to the collection tube, wherein the curable composition is formulated to have a density between an average density of the first fraction and an average density of the second fraction and to be at least one of mixable with or flowable in the liquid sample; centrifuging the collection tube containing the curable composition and the liquid sample causing the curable composition to migrate between the first fraction and the second fraction; forming, from the curable composition, a solid crosslinked composition having a hardness of at least one on the Shore 00 hardness scale using a suitable energy source.
 18. The method claim of 17, wherein the step of forming the solid crosslinked composition takes no more than 5 minutes.
 19. The method of claim 17, further comprising decanting the first fraction from the tube without contamination from the second fraction.
 20. The method of claim 17, further comprising pipetting the first section from the tube without penetrating the solid crosslinked composition. 